PURPOSE: To keep a laser-beam level at a predetermined value, and to improve accuracy on measurement while simplifying a device by comparing a detecting signal from a light-receiving section and a reference signal from a reference sig- nal generator and controlling an optical attenuator.

CONSTITUTION: Laser beams from an optical attenuator 14 are divided into output laser-beams 102 and laser beams 104 for detection by an optical branching circuit 16, and the laser beams 104 for detection are received by a light-receiving section 18, and converted into an electrical signal by the light-receiving section 18. A detecting signal 108 from the light-receiving section 18 and a reference signal 106 from a reference signal generator 20 are transmitted over a differential amplifier 26 as a comparator, and the differential amplifier 26 compares the detecting signal 108 and the reference signal 106, amplifies the result of the comparison, and outputs it as a control signal 110. The control signal 110 receives predetermined low-pass filtration action by a low-pass filter 28 and is transmitted over the optical attenuator 14, and the optical attenuator 14 controls a laser-beam level on the basis of the control signal 110.